Traditionally, a leather welt has been stitched to a shoe upper, which was in turn attached to the shoe sole, for comfort and longevity of the shoe. Today, with the advent of unit soles, uppers are attached to the sole by cement; to preserve the traditional appearance, a separate welt is attached to the unit sole around the periphery of the upper. This welt may also increase the strength of the bond between the upper and the sole of the shoe, since the welt is cemented on both sides.
The separate welt is most commonly in the form of a vinyl (or other plastic) strip, which is stitched (or cemented) to the upper, and is cemented and/or stitched to the sole. The cement is usually applied in the form of a viscous liquid to the surface of the welt, which is then placed in contact with the sole or with both sole and upper to form a bond.
It has been suggested that manufacturing costs could be reduced by precoating the welt with a solid adhesive, which could be activated to form a bond when in contact with the sole or both the sole and upper. For example, the welt can comprise a layer of a thermoplastic polymer. Such a welt can be placed in contact with the sole (and optionally the upper), and heat can be applied to soften the thermoplastic and cause it to activate and bond with the sole (and upper). This method avoids the use of dangerous aromatics and volatile cements, and reduces the amount of tooling used in shoe manufacture. Thus, it would be expected to be less expensive than the traditional process.
A precoated welt of this type (manufactured by a coextrusion process), was briefly manufactured and sold by Rextrude, Inc., in the early 1980's. This welt was found not to form a sufficiently strong bond with the shoe, however. Further, it was found that coils of precoated welt would sometimes self-adhere during transport, particularly when left for long periods in semitrucks in the hotter regions of the country. An enclosed truck bed can reach temperatures as high as 140.degree. F. in the summer in some parts of the U.S. Because of these problems, precoated welts were abandoned by the industry.